1. Field of the Invention
The present invention relates to a linear light source using a light emitting diode.
2. Description of the Prior Art
Recently, a light emitting diode has been used in a linear light source for use in the illumination of documents and the like. Such the linear light source comprises a long substrate (plate such as print-circuit-board), light emitting diodes arranged in a line on the substrate and a rod lens supported above the light emitting diodes. Such a rod lens, which has been most widely used, is a columnar lens made of glass, as disclosed in Japanese Patent Application Laid-Open No. 58-225673 (1983), Japanese Patent Application Laid-Open No. 60-129714 (1985) and the like. The columnar lens made of glass is used because it is strong in spite of its increased length, a distance between the lens and the light emitting diodes being able to be easily maintained constant, the transparency being able to be maintained at a high value, and the like.
However, in the case where the columnar lens made of glass is used as the rod lens, problems have occurred in that it is made of glass and columnar.
At first, as to the matter that it is made of glass, the rod lens made of glass shows a problem in that a width of illumination and an intensity of illumination on a surface of illumination are difficult to regulate. That is to say, a distance from the lens to the surface of illumination, such as document, it defined and then a diameter of the rod lens and a distance between the rod lens and the light emitting diodes are defined so that an appointed width of illumination of about 0.5 to 1.2 mm may be obtained. But if the width of illumination is increased, the intensity of illumination is reduced. In general, in order to make the installation of the linear light source and reading elements easy, it is required that the width of illumination be as large as possible to an extent of 1.0 to 5.0 mm. Accordingly, the intensity of illumination is reduced to nearly the permissible limit and the diameter of the rod lens made of glass, the height of a frame member for supporting the rod lens and the like must be redesigned every time when the specifications are slightly changed, so that the productivity has been inferior.
So, it was investigated to mold the rod lens from resin capable of relatively freely changing a curvature of the lens. However, the rod lens has a diameter (or thickness) of at most 10 mm and a length of 80 to 500 mm, that is it is slender, and it can not have any intermediate joints, so that the rod lens made of resin is apt to form surface patterns resulting from traces of the flow of resin during the molding and dislocation boundaries, which lead to an uneven illumination and are undesirable. In addition, if the conditions go still more worse, bubbles are formed in the lens and the transparency is reduced because of the rough surface, cloudiness and the like, whereby the desired optical characteristics can not be obtained, which have been disadvantageous. Furthermore, even if the rod lens has not taken such a disadvantage, the substrate and the frame member are apt to extend and contract on account of the temperature rise of light emitting diodes when they are turned on. A slender resin lens is not capable of correcting such an extension and contraction. The rod lens bends into or out of its frame member making it difficult to maintain its optical positional relation relative to the surface of illumination constant.
On the other hand, the columnar lens can maintain a suitable strength and the desired optical characteristics even though its length is increased. As shown in Prior Art FIG. 1, a central portion of the columnar lens 54 serves as a so-called collecting lens, the optical path is complex, and a relatively large quantity of light irradiating places other than the surface of illumination, as shown by oblique lines in FIG. 1. This means that 1 it is difficult to design the size of the diameter of the columnar lens and the position of its center for carrying out the illumination of the desired width and the desired intensity of illumination on the surface of illumination such as a document surface and 2 if the position of the light emitting diode 52 is shifted in a direction meeting at a right angle with a central axis of the columnar lens 54 (hereinafter referred to as side shifting), the beam is greatly dislocated, whereby the linearity of the incident ray on the surface of illumination is lost. Accordingly, provided that for example the distance between the top surface of the rod lens made of glass having a diameter of 5 mm and the light emitting diodes is 6 mm and the distance between the lens surface and the surface of illumination is 6 mm, the width of illumination ought to amount to 1.5 mm. But even though the light emitting diodes are arranged in line at a severity of the permissible limit of side shifting from the center of the line of .+-.50 .mu.m, actually an effective width of illumination of only about 1.2 mm can be obtained. In addition, in general, it is required that the width of illumination be as wide as possible to an extent of 1.0 to 3.0 mm for making the installation of the linear light source and the reading elements easy, but if the width of illumination of the above described columnar lens is increased, the intensity of illumination is reduced and also the side shifting of the light emitting diodes leads to the partial reduction of intensity of illumination (uneven intensity of illumination) on the defined surface of illumination.